1. Field of the Invention
The present invention relates to a fixing device, an image forming apparatus, and a fixing method.
2. Discussion of the Background
A background image forming apparatus (such as a copier, a facsimile, or a printer) fixes a toner image onto a recording medium with heat to make a copy of the image or to create a recorded medium. The toner image is fixed onto the recording medium by heating the toner image and the conveyed recording medium while the recording medium is being nipped, because the toner either melts or softens to allow the toner to permeate into the recording medium.
Japanese Published Unexamined Patent Application No. Hei 3-5779 shows an image forming apparatus including an upper pressing roller, an lower pressing roller, a main heater and a sub heater heating the upper pressing roller, a main power source supplying power to the main heater, and a rechargeable battery supplying power to the sub heater. This application further shows a charger configured to charge the rechargeable battery from the main power source, a switch to switch a electric double-layer capacitor so as to connect to or to disconnect from the heater, and a temperature sensor to sense the temperature of the upper pressing roller. This application furthermore shows a controller to control the switch switching the rechargeable battery to connect to or to disconnect from the sub heater based on comparing the sensed temperature by the temperature sensor to a threshold value of the temperature. In addition, Japanese Published Unexamined Patent Application No. Hei 2000-98799 and No. Hei 10-282821 show structures similar to those shown in JP 3-5779.
In these prior art references, the rechargeable battery can supply the power to the sub heater independent of the rating of the main power source. Thereby it is possible to rapidly heat the upper pressing roller independent of the rating of the main power source. However, these references are prone to rapid battery discharging and resultant multiple charging cycles which reduces the overall life of the battery.